Googler, JohnMu, called this the "big issue with IIS". John suspects the "newest version of IIS can handle things a bit better," but he said that most hosting companies are not running that version of IIS yet. So what options do you have?

John explains that using sessions to manage this won't work for search engines, because spiders don't handle sessions well. Thus if a spider find it, they will just run into an infinite loop on your site and that can be bad for many reasons.

John recommends the following:

The best solution is to make sure that there is absolutely no mention of "/default.asp(x)" on your site, instead only mentions of "/". You can confirm this by using a crawler such as Xenu's Link Sleuth. However, take care that you do not use forms anywhere, because they will generally return their results back to the file ("/ default.asp(x)").

For more details on this issue and troubleshooting it for your site, I highly recommend you read the whole thread.